The disclosure relates to a thermally stable optical sensor mount, such as optical encoder head used to monitor changes in the relative position of a wafer stage in a lithography tool based on an encoder scale.
A problem that is commonly encountered in the field of precision instrument design is the requirement to mount or attach two bodies of different coefficients of thermal expansion (CTE) in such a manner as to maintain the relative locations of the two bodies in the face of temperature, and accompanying dimensional, changes. In general, the mounting method has to accommodate the relative dimensional changes between the two bodies to prevent distortions, and constrain the location of a point of interest so as to prevent the motion of this point relative to a reference frame external to the two bodies.
FIG. 1 shows one possible arrangement of constraints 100 that satisfies the above requirements. Expansion of a body 105 relative to a reference frame (e.g., a mechanical reference frame) results in relative displacement between all parts of the body and the reference frame, except for one point known as the thermal center (TC). This point is located at the intersection of the lines that are perpendicular to the constraint lines. The thermal center is significant in that this is the point at which the probe (or tool) in an instrument is located so as to exploit the dimensional invariance of this point to obtain the required thermal insensitivity.
FIG. 2 shows constraint systems 200 that constrain one body relative to the other have thermal centers defined by the geometry of their constraints. Two example implementations of the kinematic mount are the Maxwell and Kelvin clamps, 210/215 and 250, respectively. The Maxwell clamp geometry 210/215 is often preferred over the Kelvin clamp 220 because the TC coincides with the center of the mount, e.g., in a part of the mount that is unobstructed. FIG. 3 shows modified Maxwell clamp geometries 300′, 300″ in contrast with the symmetric Maxwell clamp 300. The location of the TC can be modified by changing the geometry of the constraints C2 and C3, e.g., the orientation of two of the V-grooves. These mounts rely on sliding at the interfaces along a direction orthogonal to the constraint direction to accommodate relative dimensional changes between the bodies, with ideal performance being achieved only in the absence of friction at the interfaces. In the presence of friction, the behavior is less predictable and deviations from ideal behavior may be observed. The stiffness (i.e., the extent to which it resists deformation in response to an applied force) in the constraint and sliding direction is typically the same for all six points of constraint for small relative motions. For larger motions or in arrangements that eliminate or minimize friction in the sliding direction, the stiffness is essentially zero.
FIGS. 4A-4B show another approach that is often used to construct kinematic mounts 400 utilizes compliant connecting elements C1, C2, C3 between two bodies. The compliant elements C1, C2, C3 are designed so as to provide high-stiffness in the constraint direction and high compliance in the remaining translational and rotational directions. The quasi-kinematic flexure equivalent of the symmetric Maxwell clamp shown in FIG. 3 is shown in FIGS. 4A-4B. FIG. 4A shows one flexure arrangement C1, C2, C3 that produces a quasi-kinematic equivalent of the Maxwell clamp. FIG. 4B shows a top view of the constraint pattern illustrated in FIG. 4B. In each of FIGS. 4A and 4B, the respective thermal centers are shown and are located at the intersections of the (dotted) lines defining the “sliding direction” or the direction of maximum compliance. Based on the shapes of the elastic members C1, C2, C3, the stiffness in the two directions, i.e., the constraint direction kT and the sliding direction kR, are determined by the geometry of the individual compliant elements C1, C2, C3. In traditional implementations, the compliant elements C1, C2, C3 are nominally identical and have the same stiffness characteristics. Again, as in the implementation using sliding contacts, the location of the TC is determined by the arrangement of the complaint elements C1, C2, C3.